1. Field of the Invention
The present invention relates to an SDH add-drop multiplexer for adding, dropping, or passing through a signal of any channel of SDH (Synchronous Digital Hierarchy) signals in which signals of a plurality of channels have been multiplexed.
2. Description of the Related Art
In recent years, SDH (Synchronous Digital Hierarchy), which is standardized in ITU-T (International Telecommunications Union-Telecommunication Standardization Sector) and the ITU-R (ITU-Radio Communication sector), has come to be used as the standard of communication networks that are used in, for example, microwave communication systems. However, transmission standards known as PDH (Plesiochronous Digital Hierarchy) existed before this SDH was standardized.
In a PDH system in which PDH was used as the communication network standard, a signal that is to be transmitted is transmitted after first being converted to an SDH signal of, for example, an STM (Synchronous Transport Module)-1. In an SDH system in which SDH is used as the standard of a communication network, a signal that is to be transmitted is transmitted after first being converted to a PDH signal such as E1/T1.
STM-1 is one of the transmission units that are defined in SDH, and is a signal having a transmission speed of 155.52 Mbps. An E1 signal is the transmission unit in European hierarchy standards, and is a signal having a transmission speed of 2.048 Mbps. A T1 signal is the transmission unit in the hierarchy standards of North America and Japan and is a signal having a transmission speed of 1.544 Mbps.
An SDH signal that is used in this SDH system is a signal in which a plurality of channels have been multiplexed, and therefore requires the use of a total of two coaxial cables, one for input and one for output, for connecting two digital mod/demod devices. However, because a plurality of channels are multiplexed in a single signal, an SDH Add-Drop Multiplexer (hereinbelow abbreviated as “ADM”) device is necessary for adding, dropping, or passing through the signal of any channel when dropping a portion of the channels or adding new channels midway.
The following explanation regards the circuit configuration of such an ADM device of the prior art with reference to FIG. 1.
As shown in FIG. 1, this ADM device is made up from: SDH interface circuits (SPI: SDH Physical Interfaces) 3, 4, 10, and 11; SDH demapping circuits 5 and 9; SDH mapping circuits 6 and 8; cross-connect circuit 7; and PDH interface circuits (LIU: Line Interface Units) 14 and 15.
SDH interface circuit 3 receives as input an SDH signal such as an STM-1 signal from SDH signal input terminal 1; extracts the clock signal from the SDH signal of CMI (Code Mark Inversion) coding format that has been received as input and then converts to data of NRZ (Non Return to Zero) format; and finally supplies the result together with the extracted clock signal as output to SDH demapping circuit 5 of the next stage.
The signal that is received as input from the SDH signal input terminal 1 is in some cases an electrical signal and in other cases an optical signal. This electrical signal and optical signal are signals determined according to the standards of ITU-T, an electrical signal being a signal prescribed by ITU-T G.703, and an optical signal being prescribed by ITU-T G.957.
Thus, when the signal that is received as input from SDH signal input terminal 1 is an optical signal, SDH interface circuit 3 performs optical/electrical conversion instead of CMI code conversion, and supplies the data and clock that are obtained to SDH demapping circuit 5 of the next stage.
SDH demapping circuit 5 receives as input the data signal and clock signal that are supplied from SDH interface circuit 3, separates the signals of the plurality of channels that are multiplexed in this data signal, and supplies output to cross-connect circuit 7. For example, SDH demapping circuit 5 separates the signal from SDH interface circuit 3 into signals of 63 channels having a transmission speed of 2 Mbps and supplies the result to cross-connect circuit 7.
SDH mapping circuit 6 receives as input the clock signal and the digital signal of a plurality of channels that are supplied from cross-connect circuit 7; maps the digital signal of a plurality of channels based on a mapping method that is prescribed by ITU-T G.707; and supplies the result together with the clock signal to SDH interface circuit 4.
SDH interface circuit 4 converts the data signal and clock signal that are supplied from SDH mapping circuits 6 and 8 to an interface format (CMI coding format) that is prescribed by ITU-T G.703 and supplies the result as output from SDH signal output terminal 2. When the signal that is received as input from SDH mapping circuit 6 is an optical signal, SDH interface circuit 4 similarly converts to an optical signal of an interface format prescribed by G.957 and supplies the result as output.
The operations of SDH interface circuits 10 and 11, SDH mapping circuit 8, and SDH demapping circuit 9 are the same as the operations of SDH interface circuits 3 and 4, SDH mapping circuit 6, and SDH demapping circuit 5, respectively, and an explanation of these operations is therefore here omitted.
When SDH signal input terminals 1 and 13 and SDH signal output terminals 2 and 12 are used only for connection between devices, the interface standards may be original.
PDH interface circuit 15 receives a PDH baseband signal from PDH baseband signal input terminal 17 and converts to a digital signal of a format that allows processing in cross-connect circuit 7. More specifically, the PDH baseband signal that is received as input from PDH baseband input terminal 17 is a signal having a bipolar coding format, and PDH interface circuit 15 therefore extracts the clock signal from the PDH baseband signal that is received as input, converts the signal of bipolar format to a signal of unipolar format, and supplies the clock signal and signal that has been converted to unipolar format as output to cross-connect circuit 7.
PDH interface circuit 14 receives as input the unipolar signal and clock signal that are supplied from cross-connect circuit 7, converts the unipolar signal to a bipolar coding format, and supplies the result as output from PDH baseband signal output terminal 16.
Cross-connect circuit 7 is a circuit for switching paths by channel units for a signal of a plurality of channels that is received. More specifically, cross-connect circuit 7 branches (drops) and supplies to PDH interface circuit 14 a signal of a specific channel in signals of a plurality of channels that are received as input from SDH demapping circuits 5 and 9, inserts (adds) the signal that is received as input from PDH interface circuit 15 to the signals of the other channels, and supplies the result to SDH mapping circuits 6 and 8, respectively.
FIG. 2 next shows a system diagram for a case in which the ADM device of the prior art that is shown in FIG.1 is used to make up an optical ring network. As shown in FIG. 2, this optical ring network is configured by connecting the four relay stations A, B, C, and D in a ring by means of optical cables 121-124. Each of stations A, B, C, and D is formed by ADM devices 131-134 of the configuration that is shown in FIG. 1.
ADM devices 131-134 of the configuration that is shown in FIG. 1 are of a configuration that can pass PDH signals through, or add PDH signals to or drop PDH signals from SDH signals that are received as input, and when an optical ring network is to be made up by these ADM devices 131-134, the SDH signal input/output terminals need only be connected by optical cables 121-124.
In a communication network system such as shown in FIG. 2, relay stations are connected together by optical cable, but in some cases, a radio ring network is used in which the relay stations are each connected by radio lines. A communication network that uses such a radio ring network is disclosed in, for example, JP-A-2000-165391.
FIG. 3 shows a system diagram for a case in which ADM devices of the prior art that is shown in FIG. 1 are used to form a radio ring network. As shown in FIG. 3, this radio ring network has a configuration in which the four relay stations A, B, C, and D are connected by radio lines in a ring. Station A is made up from ADM device 131, mod/demod devices (MD) 135 and 136, transmitter-receivers (TR) 105 and 106, and antennas 113 and 114. Station B is made up from ADM device 132, mod/demod devices (MD) 137 and 138, transmitter-receivers (TR) 107 and 108, and antennas 115 and 116. Station C is made up from ADM device 133, mod/demod devices (MD) 139 and 140, transmitter-receivers (TR) 109 and 110, and antennas 117 and 118. Station D is made up from ADM device 134, mod/demod devices (MD) 141 and 142, transmitter-receivers (TR) 111 and 112, and antennas 119 and 120.
In the radio ring network that is shown in FIG. 3, ADM devices 131-134 have a circuit configuration for the relay of SDH signals, and in order to make up a radio network, mod/demod devices 135-142 are necessary for converting the SDH signals from ADM devices 131-134 to modulated signals and supplying these signals to transmitter-receivers 105-112 and for converting the demodulated signals from transmitter-receivers 105-112 to SDH signals. FIG. 4 shows the configuration of these mod/demod devices 135-142.
As shown in FIG. 4, mod/demod devices 135-142 are each made up from: SDH interface circuits (SPI) 53 and 54, SDH demapping circuit 55, SDH mapping circuit 56, signal multiplexer (MUX) 64, signal demultiplexer (DeMUX) 72, transmission digital processing unit (TDPU) 62, reception digital processing unit (RDPU) 70, modulator (MOD) 60, and demodulator (DEM) 68; and each subject an SDH signal that is received as input from SDH signal input terminal 51 to SDH demapping, signal multiplexing, transmission digital processing, and modulation to supply the signal from modulated signal output terminal 58; or subject a modulated signal that is received as input from modulated signal input terminal 66 to demodulation, reception digital processing, signal demultiplexing, and SDH mapping to supply the signal from SDH signal output terminal 52.
The processing in SDH interface circuits 53 and 54, SDH demapping circuit 55, and SDH mapping circuit 56 is the same as the processing in SDH interface circuits 3 and 4, SDH demapping circuit 5, and SDH mapping circuit 6, respectively, that are shown in FIG. 1, and explanation is therefore here omitted.
Signal multiplexer 64 multiplexes a data signal that is received as input from SDH demapping circuit 55 and having a transmission speed of 2 Mbps.
Transmission digital processing unit 62 subjects the multiplexed digital signal that is received as input from signal multiplexer 64 to both speed conversion for adding redundant bits (for example, error correction bits) that are characteristic to a radio interval and row conversion that corresponds to the modulation mode of modulator 60.
Modulator 60 modulates the digital signal that is received as input from transmission digital processing unit 62 and supplies the obtained modulated signal from modulated signal output terminal 58.
Demodulator 68 demodulates the modulated signal that is received as input from modulated signal input terminal 66 to convert the signal to a digital signal and supplies the obtained digital signal to reception digital processing unit 70.
Reception digital processing unit 70 receives the digital signal from demodulator 68 and subjects the signal to digital processing that corresponds to the digital processing that was performed in transmission digital processing unit 62 on the opposite side of the radio link.
Signal demultiplexer 72 demultiplexes the data signal that is received as input from reception digital processing unit 70 into a digital signal of a plurality of rows and supplies the obtained digital signal to SDH mapping circuit 56.
As described in the foregoing explanation, the ADM device of the prior art having the configuration shown in FIG. 1 has only SDH signal input/output terminals 1, 2, 12, and 13 as terminals for receiving and supplying signals that are to be relayed, and therefore, the outside provision of mod/demod devices 135-142 and transmitter-receivers 105-112 is necessary when this ADM device is used to construct a radio network as shown in FIG. 3.
However, as can be seen from a comparison of FIG. 1 and FIG. 4, when SDH signal output terminal 12 and SDH signal input terminal 51 are connected and SDH signal input terminal 11 and SDH signal output terminal 52 are connected, SDH mapping circuit 8, SDH demapping circuit 9, and SDH interface circuits 10 and 11 in the ADM device duplicate the functions of SDH interface circuits 53 and 54, SDH demapping circuit 55, and SDH mapping circuit 56 in the mod/demod device; resulting in the disadvantage of increased cost for configuring the system.
Based on the circuit configuration of the ADM device that is shown in FIG. 1, it is believed that this disadvantage can be prevented by adopting a configuration in which SDH mapping circuit 8, SDH demapping circuit 9, and SDH interface circuits 10 and 11 are independent. However, because the connections between cross-connect circuit 7, and SDH mapping circuit 8 or SDH demapping circuit 9 are signals of a plurality of rows, adopting a device in which SDH mapping circuit 8, SDH demapping circuit 9, and SDH interface circuits 10 and 11 are separate configurations necessitates many cables to interconnect devices and thus has the counterproductive result of increasing costs. In particular, constructing an optical ring network as shown in FIG. 2 results in excessive additional cable connections in each relay station and increases the cost of building the system.
In addition, data back-up may be performed for data of both an optical network and radio network to ensure data transmission even in the event of problems on the network such as disconnection of an optical cable and thus improve the reliability of a system. In an ADM device of the prior art, however, connecting a mod/demod device to SDH input/output terminals prevents the construction of an optical network, and using an ADM device of the prior art when backing up optical and radio networks therefore necessitated completely separate system configurations for each of the optical system and radio system.